Iron-containing alloys, such as different grades of steel and stainless steels, are subject to corrosion when exposed to aqueous environments. Thermally-sprayed coatings are frequently used in corrosive environments to provide wear resistance. There are many thermal spray coatings whose corrosion characteristics are superior to iron-containing alloys. The use of such wear and corrosion resistant coatings may be limited by the corrosion behavior of the substrate. This is because of the interconnected porosity which is inherently present in thermally-sprayed coatings. This interconnected porosity may allow the corrosive media to reach the coating substrate interface. An example of the problem is the use of a plasma-sprayed Cr.sub.2 O.sub.3 coating on a 300 series stainless steel substrate in sea water. This coating/substrate combination is frequently used for applications such as mechanical seals. The Cr.sub.2 O.sub.3 coating itself has good wear and corrosion resistance, but the stainless steels are susceptible to crevice corrosion. Consequently, Cr.sub.2 O.sub.3 coatings on 300 series stainless steels frequently fail in a sea water environment. The fabrication of mechanical seals from nickel base corrosion resistant alloys is expensive. Weld deposited overlays of nickel base corrosion resistant alloys on iron base alloys have both technical and cost problems.
It is an object of the present invention to provide an impervious coating for a metallic alloy substrate, such as an iron-containing alloy, a copper-containing alloy, a cobalt-containing alloy, an aluminum-containing alloy, or a nickel-containing alloy, that can be used in aqueous environments.
It is another object of the present invention to provide a process for protecting a metallic alloy from aqueous corrosion by applying an impervious coating to such alloy.
The foregoing and additional objects will become more apparent from the description and disclosure hereinafter.